Note 4 - Realized Gain (Loss) on Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Gross realized gains	$ 8,000	$ 18,000	$ 28,000
Gross realized losses	(147,000)	0	(4,000)
Other-than-temporary impairment	0	0	0
Total net gains (losses) on available-for-sale securities	$ (139,000)	$ 18,000	$ 24,000